How it Works: Oil Lubrication

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Shell Motor Oil
Commercially available motor oils are so good that it is almost unheard of that an engine's failure should be attributed to some deficiency in its lubricant. The oil may deteriorate in use, the engine's performance may deteriorate correspondingly, but unless there be a catastrophic failure the average car owner remains unaware that their chosen oil may not be doing all it should.

In the earliest days of motoring there were good oils and bad ones, and car owners were able to determine which was which from its origin. The best mineral oil came from Pennsylvania (which strangely also produced some of the worst fuel, the best gasoline coming from California and the East Indies), and if even that was not good enough - and for aero and racing engines it often was not - then the only answer was a high-smelling vegetable oil derived from castor or rape seeds.

Vegetable and Synthetic Oils

It was a late as the 1960's that castor oil ceased to be the basis of most racing oils. It was abandoned by aviators much earlier, new synthetic oils taking over the task - first in Germany, and then everywhere jet engines were under development. These synthetic oils were developed to the point where they were cheap enough to offer the ordinary motorist. The secret of vegetable and synthetic oils alike was their behaviour when very hot. Ordinary mineral oil becomes very thin and runny when hot, and then offers too little resistance to being squeezed out of bearings, allowing metal-to-metal contact.

Castor oil retained its film strength at high temperatures, but was stiff at low ones; synthetic oils were designed to remain stable over a very wide temperature range, their most daunting requirement being to withstand the heat soak in the centre bearings of a jet engine after shut-down. The chemical differences between the three classes of oils are far-reaching, but in this physical respect they differ only in degree: all oils flow more easily when hot than when cold. To put it another way, oil viscosity decreases as its temperature increases. This viscosity, or resistance to flow, may be measured in various ways: if you are a physicist or an oilman you may work in Centistokes or in Saybolt Universal Seconds.

Understanding SAE Ratings

The principal characteristics of lubricant are greasiness and viscosity; greasiness (top) lubricates by the molecules adhering to the metal, while viscosity lubricates by resisting the pump pressure and keeping the surfaces apart. The ratings were prescribed by the American Society of Automotive Engineers. These SAE ratings were little more than broad classifications allowing wide margins of error, and originally they were expressed in a single figure: an SAE 50 oil was thicker than SAE 40, for example. In those days you could use a thinner or numerically lower grade in winter than in summer - but then the chemists discovered how to make multigrade oils that were as fluid at low temperatures and as vicsous as a 40-grade when hot.

That was SAE 30W-40 oil: the higher figure showed how thick the oil remained when it was hot (at 98.9°C), while the lower figure indicated how fluid it was when starting at 0°C. However, the W (winter) viscosity was not actually measured at this lower temperature, but was theoretically inferred from measurement at 38°C. The theory was not always valid, and when the SAE finally declared that after 1968 the viscosity at zero would have to be measured actually at zero, several of the so-called 20W-5O oils that had by then been developed turned out not to earn their 20W rating at all, which is why the W disappeared from some cans.

The Viscosity Index

The more widely spaced the figures appear, the less widely does the viscosity of the oil vary. Some people think that a multigrade oil grows thicker as it gets hotter, whereas what really happens is that it does not get as much thinner as would a mono-grade subjected to the same temperature rise. A measure of this stability is the viscosity index, which is shortened as VI: the smaller the drop in viscosity at high temperature, the higher is the VI. Improving the viscosity index is one of the most important tasks of the oil chemist, and the chemical additives with which they achieve it are called VI improvers. They are one of the most important (but nevertheless only one) of the many classes of additives that go into a modern multigrade oil.

In any litre of oil and three quarters is made up of mineral base oil while the remainder consists of additives - not just VI improver but also anti-oxidants, corrosion inhibitors, detergents, dispersants, and several others. They are not necessarily made by the oil company whose name is on the can: a few big firms provide 'additive packages' for others to add to their own base stocks. They are not generally capable of lubricating: the additive package without the base stock would not be a super-oil. The additives are there because an oil has to do more than provide lubrication by reducing friction: it must also assist in cooling the engine, must keep it clean, protect it against corrosion and wear of various kinds, and act as a sealant to keep the combustion gases where they belong. The oil must even provide some sort of lubrication when it is not present!

It is a kind of residual lubrication when the oil film breaks down or has drained away - as when starting from cold, which is when most severe engine wear takes place. An electrochemical bonding of certain oil particles to the metal surfaces can take place, preserving a last-ditch film of protection that is again particularly effective in the case of castor oil. Modem engines sometimes have synthetic dry lubricants 'plated' on to critical areas, such as molybdenum disulphide on the edges of piston rings. Lubrication problems in cold starting explain the need for the fluidity of the multigrade spectrum. It ensures a quicker and more copious flow throughout the engine in starting at any temperature.

There is a critical range where cold starting is affected by low-temperature viscosity, a range that extends from -10° to -25°C, when thick oil makes the engine stiff and reluctant to reach the cranking speed necessary for combustion. Until the late 1970's a 10W oil could only manage an SAE 40 rating at high temperature, so cold starting ability involved a certain sacrifice; yet the 50 rating was thought essential to minimise consumption and wear. Consumption is of increasing importance because of the greater freedom for sustained high-speed running on the highway: if the oil thins out too much, it will too easily find its way out of the engine. Hence the importance of the VI improvers.

1920 Shell Tanker
This image is from the 1920's and shows a Shell Tanker parked outside the companies Canadian HQ.

An oil refinery at Antwerp in operation during the 1970's
An oil refinery at Antwerp in operation during the 1970's.

Oil Additives

These additives are polymeric, meaning that their molecules are large and complex, as are the molecules of most modern plastics. In fact it is not too fanciful to think of them as a kind of liquid plastic spaghetti: when added to a base oil they increase its viscosity at high temperature more than when cool, because as they grow hot these long chainlike molecules grow larger and more resistant to displacement. Thus a fairly thin oil can be used as a foundation to ensure easy starting and rapid circulation, while the VI improvers preserve its viscosity in hard high-temperature driving.

Other additives are vital to low-temperature operation. This is when oil suffers most contamination, when most corrosive wear takes place. Water, acids and other combustion products condense on relatively cool cylinder walls, drop, or are scraped off, into the crankcase and are emulsified with the oil to form sludge. Modem clean-air laws demanding positive crankcase ventilation aggravate sludging quite alarmingly, and have also led to a new phenomenon, the so-called 'mayonnaise' emulsion of condensed water and oil that forms inside rocker boxes and similar places. High-temperature problems are quite different. Diluents of the oil (not only condensates but also unburnt fuel) may be boiled away above about 80DC, but so may some of the oil itself: the volatility of the base stock is most important, for the more volatile oils tend to vaporise from the hotter engine parts.

These vapours are lost through the engine breather system, resulting in excess consumption and thickening of the remainder. The ring belt round the piston is the most critical area: the film of oil preventing metal-to-metal contact here may only be a tenth of a thousandth of an inch thick, and if it were too volatile it would too readily be boiled up and blown away-just as, if it were too fluid, it would be squeezed out by mechanical pressure. The loss of volatile fractions is not the only cause of viscosity increase in heavy-duty service. Another is oxidation, resulting from exposure of the hot oil to air in the crankcase.

The thickening effect is sometimes obscured by fuel dilution (a particularly nasty problem in diesel engines, for diesel fuel is not volatile) which does nothing to restore the oil's lubricity. Anti-oxidant additives resist this degradation, but after long use even the best of them cannot cope, and the higher the temperature the less their effect: oxidation actually doubles with every 18 degrees C rise in temperature. In some cases, the opposite condition causes trouble: a loss of viscosity can occur where there is not a trace of dilution. The most severe cases are in engines with built-in gearboxes sharing a common oil supply. In 1500 miles of typical European urban service, a 20W-50 oil may suffer an 18% loss of viscosity at 99DC in a conventional engine, while in a combined engine-and-transmission unit the loss is as high as 42%.

What happens is that the long chains of the VI improver are literally chopped up into short strands so that they are no longer effective. Sheer heavy duty in conventional engines will do the same: at the end of a 1000 km sports-car race, engine oil that was rated at 20W-50 on the starting grid will be more like 20W-40. Obviously viscosity cannot be expected to do everything. Gears bring tremendous pressures to bear on the oil film interposed between them, and for this there are certain so-called extreme pressure additives that play an important part in oils meant specifically for gear- boxes, axles and the like. Unfortunately these additives are unsuitable for use in heat engines, because the conditions of duty make them increase corrosive wear.

Valve-Gear Wear

The alternative used to be to rely on high film strength, but the nature of valve-gear wear problems brought about something of a revolution in lubricating techniques that relied on yet another class of additives. In the modern engine the greatest wear problem is presented by the valve-operating mechanism, with tremendous tearing, scuffing and pitting occurring at rocker tips, cam noses, tappet surfaces and similar critical areas. Scuffing, which affects cam lobes and tappets in particular but cylinder bores as well, is what happens when the oil film breaks down to allow metal-to-metal contact; and to keep it at bay demands high chemical reactrvrty in the oil to prevent local welding and tearing of the metals.

Pitting, which particularly affects valve lifters, is a different process, a fatigue phenomenon that is believed to result from deformation of the surface of the tappet, causing it to break away in small flakes. Many car manufacturers specify some kind of protection from pitting and scuffing, which the oil companies achieve by use of very specific additives that create when under pressure a kind of metallic soap or 'intermediate metallic compound' on the critical metal surfaces. This is formed by reaction with constituents of the additive package, usually organic phosphate compounds: the popular one for preventing scuffing is known as ZDDP for short, zinc dialkyl dithiophosphate.

Mechanical Tappets

Unfortunately the use of these phosphates tends to debase the oil's anti-wear properties in thin-film or boundary conditions, and is detrimental to corrosive wear too. Protection from corrosive wear is seldom specified by car manufacturers; yet it probably accounts for the majority of wear in cylinder bores and on piston rings as the result of deposits, fuel, combustion condensates, and other contaminants settling on the bores near the top of the piston stroke. However, the most crucial place for rust and corrosive wear inside older engines are the hydraulic tappets, and if a car manufacturer makes a special issue of corrosion protection it is sure to have something to do with these hydraulic lifters.

Manufacturers' stipulations for anti-wear properties are usually well defined, but again the anti-scuff treatments create problems elsewhere. It could for example be a matter of having to choose between having cam lobes worn by scuffing or having the oil-pump gears worn because of the reduction in thin-film strength. This is typical of the compatibility problems that arise in formulating an oil: it must not fight with metals, seals, gaskets, fibre gears, nor any of the other constituent materials of the engine, even including the paints and resins used for closing the pores of castings. It must also keep them all clean, and do it manifestly since this is something that the customer can see for themselves, however much he may be in the dark about the oil's other functions.

Keeping The Inside of the Engine Clean

Maintaining cleanliness involves two processes. One is the inhibition of any tendency to form deposits, the other is the maintenance of these deposits in suspension. Inhibition is the preferred action, so as to catch what are called deposit: precursors-half-burnt stuff blown past the piston rings into the sump, especially during the compression stroke, and all the acids, water and other foreign matter drawn in from the atmosphere or produced from combustion, which tend to form sludge in low-temperature operation (stop-go traffic, short local journeys, etc) or varnish in high-temperature, high-load work. Keeping particles of all these alien substances separate and in suspension in the oil is the job of the dispersant additives.

The detergent additives keep the metal surfaces clean by a process of preferential adherence, but like practically all the others they are gradually consumed or worn out by their work, constituting as they do a sort of sacrificial safety barrier. It therefore pays the motorist, as well as the oil companies, to change his car's oils much more frequently than the car manufacturer specifies as necessary. Like anti-scuff reagents, detergents can degrade the anti-wear properties of the base stock, so once again a lot of care and skill is needed in-order that a given blend of base stock and additives be successful.

Deposits and organic phosphates clash with other additives in another very important area, in which engine performance may be impaired or even suddenly and drastically curtailed: combustion-chamber deposits are very dependent on the composition of the oil. As ever, that additive package is a mixed blessing. Its most effective anti-rust ingredients are based on metals, notably compounds of calcium (though there are many other metallic compounds present, based on barium, molybdenum, tungsten and zinc), and these leave deposits in the combustion chamber that can damage the engine if a lengthy spell of stop-go traffic work be followed by a burst of high power output.

These deposits then become incandescent; and even if they do not cause gross pre-ignition (which distorts valves and burns holes in pistons) they will cause enough to reduce power, induce overheating, and accelerate wear. Some recent work on this problem led to the substitution of magnesium sulphonates for the traditional calcium compounds. They leave less weight of deposits, which in any case tend to soften rather than to glow and are therefore less likely to cause pre-ignition. In fact the likelihood of destructive effects, which increases with what is called the ash content of the oil, has been shown to be no greater with a magnesium- based ash content of 1-4% than with calcium-based ash of 0.5%.


Most high-grade engine oils on the market have ash contents of about 1%, though some have as little as 0.65%. In oils developed specifically for racing, the ash content may be brought as low as 0.36%. Racing oils of the vegetable type are simply not suitable for street use, however. Oxidation is the main bugbear, creating dreadful deposits under piston crowns and around valves. Rust-inhibitors have adverse effects that rule them out, and in the end only the high film strength of castor oil remains to its credit. The synthetic oils are a different matter: their constituents are chosen rather than determined by nature, and certain varieties may be highly recommended for use in road cars.

There have been snags in the past, though: a French company put a synthetic oil on the market a few years ago and ran into terrible trouble because it reacted disastrously with the slightest trace of conventional mineral oil in the engine. More recent synthetics are completely and safely miscible with conventional lubricants, and are merely very expensive. Racing has a few problems of its own. One is that ZDDP forms new chemical structures in the presence of alcohol fuel, leaving nasty deposits and causing drastic wear on certain surfaces. Another is that the very high rates of flow in a racing engine cause frothing and foaming of the oil" aggravated by the fact that the scavenge pumps of a dry-sump racing engine have a much higher capacity than the pressure pump, so that air is entrained with the oil.

The antidote is a tiny admixture of a short-molecular-chain silicone oil, such as is commonly added to suspension damper fluids to check aeration; and this is generally present in commercially available oils, though in smaller dosages. On the whole, oils for touring cars have a much harder life than anything in racing. To deal with all these problems the' chemists formulating a new oil may spend as much as two years on screening and testing its ingredients. A suitable base stock has to be chosen from what is available-itself a knotty problem now that the choice of petroleum stocks is influenced by political and economic considerations -to have the appropriate qualities of lubricity, volatility range, and so on.

The VI improvers and all the other additives have to match it and be compatible with themselves and everything else, and even the nature of fuels likely to be used has to be taken into account. For example the presence of tetraethyl lead and related anti-knock compounds has a marked effect on combustion chamber deposits on the one hand, and on valve lubrication on the other. The lead acts as a lubricant for the valve faces and seats (and stems, though this is less critical) by coating the critical areas with lead salts formed during combustion. In their absence, particles of iron and iron oxides emerge from the material of the valve seats and, being harder than the surrounding metal, are hammered in to form craters around each nodule: the valve gradually sinks into its seat, at a rate that can reach a tenth of a millimetre an hour in a cheaply produced cast iron head with no separate valve seats.

Of course there are more kinds of lubrication in a car than are to be found in the engine, but the others are less beset by problems. High-melting-point greases may be necessary for hub bearings, to avoid the danger of liquefied? grease flowing out of the hubs into the brakes. Solid lubricants such as molybdenum disulphide or graphite may be added in large doses to greases for suspension or steering. bearings so that friction is kept low even when bearing pressures force the bulk of the grease away from the mating surfaces. Many bearings for suspension elements, steering joints, and body hinges are nowadays made of plastics that have very low coefficients of friction on dry metal and therefore need no lubrication at all - or they may be loaded or impregnated with molybdenum disulphide to improve their performance.

Plastics often have peculiar frictional properties: nylon is lubricated by water, for instance. So is rubber, and the design of water pump seals may have to take this into account. Waxy additives in coolant antifreeze may ensure the lubrication of water pump glands, while other seals for the retention of oils in the engine must be made of appropriate synthetic rubbers that are not attacked by hydrocarbons as natural rubber is. In fact tribology - the science of mutually rubbing surfaces-is develop- ing at a great pace, and the art of lubrication is undergoing a profound metamorphosis that makes the detail of a modem car very different from that of one built ten years ago. Engine lubrication will remain the most difficult for the foreseeable future, however, and is the aspect in which neglect will prove most costly to the present car owner.
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